Pandas column flatten into duplicate rows

Question

I have a dataframe with one column as a string, which has values separated by commas. I want to flatten this into a column with only single string values with other columns becoming duplicates

df = pd.DataFrame({'a':['1,2','4','3,5'], 'b':['a','b','c'], 's':[.1,.2,.3]})

Which gives a dataframe like so:

       a        b       c
      '1,2'    'a'     .1
      '4'      'b'     .2
      '3,5'    'c'     .3

I want to turn this into a dataframe that looks like:

df = pd.DataFrame({'a':['1','2','4','3','5'], 'b':['a','a','b','c','c'], 's':[.1,.1,.2,.3,.3]})

like:

       a      b       c
      '1'    'a'     .1
      '2'    'a'     .1
      '4'    'b'     .2
      '3'    'c'     .3
      '5'    'c'     .3

I have attempted to start doing this by splitting the string columns

df = df.join(df[a].str.split(',', 1, expand=True))

which appends the split string column into new columns on the end, but I'm at a loss to finish the task. Any help is appreciated!